FIG. 18 is a diagram showing a prior art pulse noise detecting circuit in an AM receiver Note, the same reference numerals and symbols are used to represent the same elements throughout all of the drawings. The construction of the figure includes a tuner 100 of the AM receiver, a pulse noise removal portion 2 for removing pulse noise from an output signal of the tuner 100, and a pulse noise detecting portion 40 for detecting the pulse noise to output a gate signal for removing the pulse noise to the pulse noise removal portion 2.
The tuner 100 includes an antenna 101, a RF amplifier 102 connected to the antenna 101, a mixer 103 connected to the RF amplifier 102, a local oscillator 104 supplying a mixing signal to the mixer 103, a broad band filter 105 connected to the mixer 103, an intermediate frequency amplifier 106 connected to the broad band filter 105, a narrow band filter 107 connected to the intermediate frequency amplifier 106, and, for example, an envelope detector 108 connected to the broad band filter 107. Automatic gain control is performed using the output of the detector 108 to the amplification gain of the intermediate frequency amplifier 106. The output of the detector 108 is made constant.
The pulse noise removal portion 2 includes a gate circuit 21 and a holding capacitor 22.
FIG. 19 is a view illustrating the removal of noise using the pulse noise removal portion of FIG. 18.
As shown in (a) of the figure, in a signal input to the pulse removal portion 2, pulse noise is superposed on a detection signal. This pulse noise is, for example, ignition noise, noise mixed from a transmission line, etc. in a receiver installed in a vehicle. Also, as shown in (b) of the figure, there is a gate signal detected by the pulse noise detecting portion 40 and synchronized with the pulse noise. Further, as shown in (c) of the figure, when the gate signal allows the gate circuit 21 to be open and the pulse noise to be removed, the level of the detection signal before removal of a pulse noise signal is held by the capacitor 22. When the gate circuit 21 is closed, the holding of the detection signal is released As shown in (d) of the figure, the post removal waveform shown in (c) of the figure is corrected by another apparatus not shown.
The pulse noise detecting potrion 40 of FIG. 18 includes an amplifier 41 connected to the output of the broad band filter 105, an envelope detector 42, for example, connected to the amplifier 41, a high pass filter 43 connected to the detector 42, and a level detecting portion 44 connected to the high pass filter 43 and producing a gate signal for operating the gate circuit 21.
The reason why the tuner 100 is provided with the broad band filter 105 is to obtain a greater amplitude of noise in the pulse noise detection portion 40 so as to make the detection of the pulse noise easier.
Also, in order to make the output of the detector 42 constant, automatic gain control is performed to control the amplification gain of the amplifier 41.
Next, the operation will be discussed. A received signal with pulse noise mixed is branched by the band filter 105, passes through the amplifier 41, has the carrier wave removed by the envelope detector 42 to become a demodulation wave, has all but the pulse noise high frequency removed by the high pass filter 43, is shaped into the gate signal by the level detecting portion 44 and is then output to the gate circuit 21.
However, since the prior art pulse noise detecting circuit of the AM receiver is required to amplify the branched signal in the band filter 105 and further perform detection etc., there were the problem of emission of a higher harmonic wave components of 910 KHz caused by detection distortion of an intermediate frequency 455 KHz after the mixer 103 and issuing of a beat ("tweet"), the problem of a need for special consideration in the layout of parts for preventing the higher harmonic wave from being mixed, and the problem of a low freedom of design owing to the limitations in construction of the band filter 105.
Therefore, the present invention, in consideration of the above problems, provides a pulse noise detecting circuit of an AM receiver in which the freedom in design is higher and malfunctions are improved.
Also, in the prior art pulse noise detecting circuit of the AM receiver, the next problem arises.
FIG. 20 is a view showing the construction of the level detecting portion of FIG. 18. The figure includes a comparator 441 whose non-inversion terminal inputs the output signal of the high pass filter 43 and whose inversion terminal inputs a referrence voltage +V.sub.r a comparator 442 whose inversion terminal inputs the output signal of a high pass filter 43 and whose non-inversion terminal inputs a reference voltage -V.sub.r, an OR circuit 443 connected to the comparator 441 and 442, and a monostable multivibrator 444 connected to the OR circuit 443 and outputting the gate signal to the pulse noise removal portion 2.
Next the operation of the level detecting portion 44 will be discussed.
FIG. 21 is a view illustrating the signal waveform at the level detecting portion 44 of FIG. 20. (a) of the figure shows a waveform of the signal output from the detector 42, and (b) of the figure shows a waveform of a signal from the high pass filter 43 extracting high frequency noise components from the signal of the detector 42, which differs from the original waveform of the output signal of the detector 42. (c) of the figure shows the output signal of the comparators 441 and 442. It is a positive rectangular pulse produced at more than +V.sub.r when the signal is positive and less than -V.sub.r when negative. (d) shows a waveform of the output signal produced by the monostable multivibrator from the comparators 441 and 442 through the OR circuit 443.
(a) of the figure shows when the pulse noises is large and small. The wave width of the pulse noise, however, increases as the wave height thereof increases. Further it is determined fundamentally according to an impulse response of an intermediate frequency filter of the tuner 100. Various kinds of distortion are caused and the wave width spreads if the pulse noise added becomes large. The places causing the distortion are the intermediate frequency amplifier 106, the amplifier 41, the detectors 108 and 42 etc. Therefore, when the positive and negative side pulses are detected and the pulse width of the gate is made by the monostable multivibrator 444, since the pulse width changes depending on the pulse height, usually, as illustrated, it is optimized to the higher value of the pulse height. Thus said .tau..sub.B is made .tau..sub.1 +.alpha.. Here, .tau..sub.1 is a time interval between the positive and negative sides and .alpha. is a design margin. In this case, even if there is no longer any pulse noise, the gate is open during .tau..sub.B =.tau..sub.1 +.alpha.. A problem arises in that in the case of a small wave height of the pulse noise the gate width is too broad and an excess of wave distortion occurs as shown in FIG. 19(c). Further, the provision of two comparators which is required to detect the positive side pulse and the negative side pulse introduces an obstacle for simple construction.
Further the present invention, in view of the above problem, has as its object to provide a pulse noise detecting circuit of an AM receiver varying the gate width according to the wave height of the input pulse.